Renal tubular acidosis (RTA) is a buildup of acid in the body resulting in a metabolic acidosis with a normal anion gap due to improper and dysfunctional acidification of the urine in the kidney tubules. RTA and diarrhea are two of the most important conditions involving a normal anion gap with a metabolic acidosis. The normal value of anion gap in both diarrhea and Renal tubular acidosis is because in both these disorders, the chloride level compensates and rises to normalize the anion gap. Thus they are also referred to as hyperchloremic metabolic acidosis.
Types of Renal tubular acidosis
- Distal RTA
- Proximal RTA
- Type 4 RTA
The distal tubule excretes hydrogen ions via producing bicarbonate and disposing them off through an exchange mechanism. In Distal RTA, this function to produce bicarbonate is lost or damaged due to a number of possible causes including autoimmune disorders and drugs like lithium. Thus the resultant urine produced, is basic.
Diagnostic test is rather crude. We give acid to see if there is a fall in the urine ph. Ammonium chloride is utilized for this purpose. Normally, the ph should decrease but in distal RTA , it (urine ph ) remains basic, that is ,over 5.5.
In such an alkaline urine, there is an increased risk of calcium oxalate stones being formed and can lead to nephrocalcinosis.
The treatment is simply to replace the bicarbonate at the proximal tubule which is intact and would counter the acidosis and correct it.
Proximal tubule is responsible for the majority of bicarbonate reabsorption which it cannot do so in Proximal Renal tubular acidosis. Initially there is basic urine due to all the bicarbonate being lost till the body becomes so depleted that the distal tubule starts absorbing the rest of the bicarbonate leading to an acidic urine.
Thus this serves as a diagnostic test, too. Give sodium bicarbonate IV and look for test the urine ph. At low serum bicarbonate levels, normally people don’t excrete bicarbonate in their urine. But in Proximal RTA, even in acidemia the urine is initially basic due to dumping of all the bicarbonate initially and then ultimately getting acidic.
The chronic metabolic acidosis, predisposes patients to bone disorders such as osteomalacia. It is also associated with rickets.
Too much bicarbonate would be required for treatment and would not be logical to keep on replacing such high losses. Treatment is through diuretics like thiazides which cause a volume depletion that would increase bicarbonate reabsorption.
Type 4 Renal tubular acidosis
Type 4 RTA mainly occurs in diabetic patients. It is a state of low renin and low aldosterone.. The low aldosterone state leads to decreased sodium reabsorption and greater levels of potassium and hydrogen. Hyperkalemia is a an evident feature/ clue on investigation in such cases..
Diagnosis is a by detecting an abnormally high urine sodium despite a low sodium diet or sodium restriction.
Treatment is replacement of aldosterone by administering fludrocortisone which has similar mineralocorticoid effect like aldosterone.
A related article to this section explaining anion gap in serum and urine is Urine anion gap calculation and utility